Pollen season in the United States stretches from late winter through the first hard frost of fall, covering roughly eight to ten months depending on where you live. In the warmest parts of the country, some level of pollen is in the air nearly year-round. The season isn’t one continuous wave, though. It comes in three distinct phases: trees, grasses, and weeds, each with its own peak and timeline.
The Three Phases of Pollen Season
Tree pollen kicks things off first, running from February through April in most of the country. Some regions, particularly the South and parts of the Southwest, see tree pollen as early as December or January. Species like juniper, maple, and elm are usually the first to release pollen, followed by pine, oak, hickory, and ash. Production ramps up sharply by mid-March, with the peak hitting at the end of March into the first week of April before tapering off through May.
Grass pollen takes over next, from roughly April through early June. This is the phase many people associate with “spring allergies,” since it overlaps with the tail end of tree pollen season, creating a one-two punch in April and May.
Weed pollen, dominated by ragweed, fills the rest of the calendar. Ragweed starts blooming in mid-August across the East and Midwest (a bit later in the South) and continues until the first hard frost kills the plants. Ragweed is considered the most allergenic of all pollens, and its effects can linger for weeks after the pollen itself disappears from the air. If you take allergy medication for ragweed, it’s worth continuing for several weeks past the first frost.
How Location Changes the Timeline
Geography is the single biggest factor determining how long your pollen season lasts. The key metric is your area’s freeze-free season, the stretch of consecutive days when overnight temperatures stay above 32°F. Plants can’t produce pollen during freezing conditions, so longer freeze-free seasons mean longer exposure.
In the southern United States, the average freeze-free season runs about 231 days per year. In the Pacific Northwest, it’s closer to 168 days. That’s a difference of more than two months. Southern cities can see tree pollen as early as December and weed pollen lingering into November, while northern and mountain regions typically have a tighter window from March or April through September or October.
Pollen Seasons Are Getting Longer
If pollen season feels worse than it used to, the data backs that up. A major study published in the Proceedings of the National Academy of Sciences found that pollen seasons across North America have lengthened by about 20 days on average since 1990, with pollen concentrations increasing by 21%. Spring pollen is arriving roughly 20 days earlier than it did three decades ago, driven primarily by warmer temperatures linked to climate change.
The trend is showing up city by city. An analysis of 198 U.S. cities found that 87% of them saw their freeze-free growing season lengthen from 1970 to 2025, by an average of 21 days. The Northwest has seen some of the most dramatic shifts, gaining an average of 24 extra frost-free days over that period. The South, already warm, added about 14 days. Higher temperatures, changing rainfall patterns, and increased carbon dioxide levels all contribute to plants growing more vigorously and producing more pollen over a longer window.
How Weather Shifts Daily Pollen Levels
Rain has a complicated relationship with pollen. A light, steady rain washes pollen out of the air and the humidity afterward helps keep counts low. But heavy downpours and thunderstorms can actually make things worse. Raindrops slam into pollen clumps on the ground, shattering them into smaller particles that scatter rapidly into the air. This can cause a sudden spike in allergy symptoms during or right after a storm, especially during grass and weed pollen season.
Time of day matters too, though not in the way most people assume. Pollen counts tend to be lowest between 4:00 a.m. and noon. Levels climb through the afternoon, peaking between 2:00 and 9:00 p.m. If you’re planning outdoor exercise or yard work, morning is your better bet.
Mold Adds to the Overlap
Pollen isn’t the only outdoor allergen on a seasonal schedule. Mold spores thrive on decaying plant material and are present outdoors year-round, but outdoor mold levels spike in the fall. That means mold season overlaps directly with ragweed season, compounding symptoms for people sensitive to both. If your allergies seem to worsen in September and October beyond what ragweed alone would explain, mold exposure is a likely contributor.
A Month-by-Month Overview
- January to February: Tree pollen begins in southern and coastal regions. Most of the country is still clear.
- March to April: Tree pollen peaks nationwide. Grass pollen starts ramping up by late April.
- May to June: Grass pollen peaks. Tree pollen fades.
- July: A relative lull in many areas, though grass pollen can persist and mold begins rising.
- August to September: Ragweed dominates. Mold spores climb.
- October to November: Ragweed tapers off after the first hard frost. Mold peaks on fallen leaves and decaying vegetation.
- December: Most of the country gets a break, though the southernmost regions may already be seeing early tree pollen.
For most people in the U.S., the realistic window of pollen exposure runs from sometime in February or March through October or November. That’s roughly eight months. In warmer climates, it can stretch to ten or eleven. And with each passing decade, those windows are creeping wider.